Growing wings

ISRO crosses yet another milestone with the successful launch and return of a winged Reusable Launch Vehicle in a scaled-down configuration that flew in a hypersonic regime.

BRINGING down significantly the cost of access to space is a primary goal of space programmes around the world today. What this means, for example, is reducing the cost/kg of payload delivered when a launch vehicle is used to put a satellite into orbit. There are, of course, different options by which one can think of reducing this cost from the following basic considerations. Typically, the on-board propellant accounts for about 85 per cent of the mass and the rest is structural hardware. The payload fraction itself is quite small, ranging from 0.4 per cent to 1 per cent of the total mass depending upon the mission and the efficiency of the system. In terms of cost, however, the hardware accounts for about 80 per cent of the launch cost. The cost of the fuel itself is negligible.

Conceptually, therefore, there are three strategies to reduce the cost/kg of payload mass: (i) improve the payload fraction by adopting newer technologies and improving the overall efficiency of the system, including increasing the thrust of the vehicle by more efficient liquid fuel engines, such as semi-cryogenic and cryogenic engines; (ii) recover the hardware and reuse for multiple launches; and (iii) reduce the initial propellant loading by adopting newer combustion modes such as air-breathing ramjet/scramjet propulsions, and any suitable combination thereof depending on the mission at hand.

One concept that has been widely worked on in many countries is that of Reusable Launch Vehicles (RLVs), which is essentially strategy (ii) mentioned above. That is to say if, instead of using, as is the current practice, a once-through Expendable Launch Vehicle (ELV)—usually a multistage rocket system—that delivers the satellite into the desired orbit and is discarded, the launcher can be brought back to earth in recoverable form in its entirety, or at least some stages of it, and used again for more launches, thus saving on the huge hardware cost.

Even though such RLVs have been discussed widely in the literature, and many countries had embarked into RLV programmes since the 1980s, a viable technology that is fully reusable and truly brings down launch costs is yet to emerge in the global launch market, though this may be changing with the advent of the recent innovative launches by the Space Exploration Technologies Corporation (SpaceX), a private enterprise started by the young American millionaire Elon Musk.

By and large, the approach to the RLV has been based on the concept of a spaceplane, a winged configuration aerospace vehicle as the launcher which, after completing its task, will “fly” back to the earth and land horizontally, much like an aircraft. America’s space shuttle and Russia’s Buran, both now discontinued, are perhaps programmes that came closest to achieving reusability in this form. However, both were only partly reusable, requiring, as they did, extensive refurbishing after every launch. The Shuttle programme was shut down in 2011 following tragic mishaps and its questionable economic viability. The Buran programme was officially cancelled in 1993 following the collapse of the Soviet Union.

The Indian Space Research Organisation (ISRO), too, has been pursuing studies on RLV technology towards achieving a two-stage-to-orbit (TSTO) launch capability based on an RLV. Basically, conventional launch vehicles are multistage. While the efficiency of the vehicle increases if there are a larger number of stages, the reliability comes down because of the multiple ignitions and stage separations required. To increase reliability, and to bring down costs, the aim is to minimise the number of stages. Although single-stage-to-orbit (SSTO) can be the ultimate goal, SSTO does not seem feasible yet with the available technology. This TSTO vehicle can be totally expendable or totally/partially recoverable.

Successful launch ISRO’s RLV programme crossed a milestone on the morning of May 23 when it successfully launched and returned a technology demonstrator of a winged RLV (RLV-TD) in a scaled-down configuration that flew in a hypersonic regime. Clearly, this has given the RLV programme a big boost. Flown from the Satish Dhawan Space Centre (SDSC), Sriharikota, the experimental flight called HEX-01 was chiefly to test the RLV-TD’s hypersonic re-entry characteristics and capabilities. (When the speed of a body exceeds the speed of sound of 343 m/s, or breaks the sound barrier, the various speed regimes are characterised by a Mach number, which is the ratio of the speed of a body to that of sound. It is called supersonic when it exceeds Mach 1 and it is hypersonic when it is Mach 3-5.)

In ISRO’s conceptual plans for achieving TSTO capability, such a winged RLV would form the base or the first stage. It would be powered by an “air-breathing propulsion” system, using a dual mode ramjet/scramjet engine that can perform over a wide Mach number range, in a subsonic-to-supersonic-to-hypersonic regime.

What is air-breathing combustion? In a conventional launch vehicle such as the Polar Satellite Launch Vehicle (PSLV, which carries about 300 tonnes of propellants), about 80 per cent of the propellant is consumed in the atmospheric phase itself. And out of the total propellant loading, about 70 per cent is the oxidiser. In air-breathing propulsion, air (oxygen) is taken in as the launcher system traverses the atmosphere. Thus, by avoiding carrying an oxidizer along with the fuel, an air-breathing-engine-powered launcher can, in principle, yield a higher payload fraction. Also, when we are talking of air-breathing propulsion, turbojet engines, etc., can at best attain Mach 1.5-2 in a high dynamic pressure regime. But aerospace vehicles fly in highly rarefied atmospheres; they have to go up to 90-100 km. So only ramjet (for lower Mach regimes) or scramjet (for higher Mach regimes) propulsion can work.

In the RLV-based TSTO, the first stage would carry the second stage (with the payload) high up into the atmosphere from where the second stage would separate and go to the desired space orbit using conventional rocket propulsion, say a cryo-engine, and deliver the payload. While after separation the first stage would fly back like an aircraft, the second stage too could be brought back by appropriate mechanisms depending on the mission if necessary, as it was done for ISRO’s Space Capsule Recovery Experiment (SRE).

The ultimate objective of ISRO’s RLV programme is to enable the vehicle to traverse a very wide range of flight regimes, from Mach 0 to Mach 25, based on air-breathing propulsion for achieving TSTO launch capability for low-earth orbit (LEO)—up to 200 km—missions. However, according to K. Sivan, Director of the Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, the nodal ISRO unit for launch vehicle technologies, this original thinking is slowly changing. “Now also we are talking of a TSTO vehicle, but it is a much bigger vehicle like a heavy-lift vehicle,” he said.

For this technology validation flight HEX-01, however, the RLV-TD (measuring 6.5 m) was mounted atop a long (10.5 m) solid booster rocket (Fig. 1), called HS9 (which carries nine tonnes of the propellant). The objective of the experiment was to achieve hypersonic speeds to test the hypersonic aero-thermodynamic characterisation of the winged body’s re-entry; its control and guidance systems; autonomous mission management to land at a specific on-sea location; the Thermal Protection System (TPS) designed to withstand the high temperatures; testing of “hot structures”, such as carbon-carbon composites, that make up the structure of the RLV; and validation of Flush Air Data System (FADS). In this experimental flight, the RLV-TD was meant to reach hypersonic speeds with the help of the booster alone and the RLV did not have any powered flight of its own.

In conventional launch vehicles, the structural elements are called “cold structures” because temperature is not a criterion; only structural strength is important. In RLVs, however, thermostructural stability becomes more important. In fact, RLV structures undergo thermostructural tests as compared with structural or thermal tests for normal launch vehicles. The structural elements of an RLV are, therefore, called “hot structures”. The nose-cap of the RLV-TD, for example, is made of carbon-carbon composites. “This is going to be our future hot structure,” explained Sivan. “All our future vehicles will involve either C-C or carbon-matrix composites. These are the new exotic materials that are going to be used in space technology in the future,” he added.

The C-C composite in the RLV nose cone is different from what was done for the SRE. “In the SRE, it is not C-C. It was carbon-phenolic tiles, an ablative surface. When we are talking of RLVs, we cannot have ablative materials. This is one reason why the shuttle programme failed. Once there is ablation, the shape of the vehicle itself will change,” Sivan pointed out. While in this experimental launch, tiling was used for the belly portion, future launches will not use tiling, he said. “We will use C-C or reusable metallic thermal protection systems. This is a new technology that is emerging.”

Test bed for validation FADS is another new technology that ISRO has developed, which the RLV-TD has used as a test bed for validation. “In a conventional aircraft, air data measurements such as air speed and angle of attack are done with the help of vanes, etc., which project out of the vehicle. Those systems are subsonic systems, which can get burnt off in an RLV under high temperatures. So we have to ‘flush’ these with the body so that there are no protrusions, no probes, no vanes, etc. Based on surface wind pressure measurement by a set of pores in the body, an on-board algorithm computes data like wind speed, angle of attack, and other parameters,” explained Sivan.

After lift-off, which was at 0700 hrs, there was booster powered flight for 91.1 s (altitude about 30 km) at which point the solid booster burnt out. The two stages together coasted for another 20 seconds, reaching an altitude of about 56 km when HS9 separated and fell off (see flight sequence, picture 4). The RLV alone further coasted to reach a peak altitude of 65 km, which marked the beginning of the descent phase. During its descent, it re-entered the atmosphere when it attained the peak velocity of about Mach 4.8 (about 1.6 km/s).

According to the ISRO release, “the vehicle’s Navigation, Guidance and Control system accurately steered the vehicle during this phase for a safe descent and landing on sea”. After surviving the high temperatures during re-entry with the help of its TPS, the RLV-TD successfully landed over the Bay of Bengal at a distance of 412 km from Sriharikota, about 13 km from the predesignated spot (Fig. 2 and 3). The total flight duration from launch to landing was about 770 seconds (about 13 minutes).

“The orientation of the vehicle or the ‘angle of attack’ was changed from 1 to 24 after separation,” N. Shyam Mohan of the VSSC, the RLV-TD Project Director, informed Frontline . “This was followed by the controlled descent through the predefined regime of velocities and orientations, satisfying the load, temperature and other aerodynamic constraints,” he said.

“The RLV-TD configuration in this mission is unique,” he said, “because the double delta wing body spaceplane was placed on top of the long and slow burning solid booster, which was specially designed for this purpose. This configuration was known to have high instability, which was efficiently managed by state-of-the-art control design. Four large fins in cruciform, other control surfaces and thrusters provided the necessary controls in a perfect way in both the ascent and descent phases,” he said.

“No one else in the world has tried this configuration where the winged body was exposed on top of the rockets. All similar winged body flights were done by NASA [National Aeronautics and Space Administration] and the ESA [European Space Agency] by putting the winged body inside a heat shield. We have done this configuration to capture the hypersonic aerothermodynamics’ characterisation of the double-delta-winged body, which is an ideal candidate for a reusable launch vehicle,” Shyam Mohan said.

The mission was also “unique” because it was designed with the least cost implications to get the maximum data in a single mission itself. Even though hypersonic flight characterisation was the prime objective of the mission, ISRO demonstrated the vehicle’s re-entry and landing manoeuvres. As many as 968 measurements done on the spaceplane have given ISRO a wealth of data that makes it confident of the winged RLV technology.

While the post-flight analysis (PFA) of the experimental launch is currently on, “quick look data show that whatever was predicted has been borne out very well”, said Sivan (see Table on “Mission parameters” and accompanying interview with ISRO Chairman A.S. Kiran Kumar). “The following key aspects of the design were successfully demonstrated in the test: understanding the hypersonic aerothermodynamics of the delta-winged body, its TPS, the hypersonic re-entry and autonomous navigation and landing and the FADS. The TPS covering the entire winged aerospace vehicle, which shielded the vehicle during its re-entry into the atmosphere, worked well. The ‘hot structures’ have remained intact. The winged vehicle’s nose-cap, covered with carbon-carbon composites, withstood the high temperatures.”

“One concern we had was about the control surfaces providing effective aerodynamic stability. When the vehicle passes through different flight regimes, it will experience different dynamic pressures. So different levels of force should be available through the control system. This can sometimes destabilise the vehicle instead of controlling it. But all the systems, including control surfaces, have performed very well. In a first-time launch, usually a lot of dispersion [in parameters] will be there. But here the dispersions seen were small and well within the expected range. We have achieved what we set out to do,” he added.

FADS validated In this experimental launch, however, FADS was not used in an operational mode. It was only used for monitoring. The real-time air data values were being inferred through on-board algorithms from a priori inputs of approximate values of wind speed, etc., for standard atmosphere pressure values at different altitudes and parameters measured by the inertial guidance system sensors. According to Sivan, data monitored by FADS matched quite well with the air data computed otherwise. “Now that FADS has been validated, we can use it in operational mode for the next RLV flight,” Sivan said.

The mission “demonstrated a lot of technologies that we need for realising a really reusable launch vehicle system”, Sivan said. “The data from the mission are a treasure,” he added, and said that they would be analysed in the coming weeks. “A lot more technologies, like air-breathing propulsion, have to be developed to build a really reusable launch vehicle system, which we will be concentrating on.” According to him, ISRO will be testing its air-breathing propulsion system, which will go up to Mach 6-8, in June at Sriharikota.

In this ensuing air-breathing test, a small scramjet engine will be hitched to a (560 mm diameter) sounding rocket to reach Mach numbers 6-8 by igniting the engine with dynamic pressure of 80 kilopascals (100 kPa = 1 bar). “What we wish to demonstrate is hypersonic ignition and sustainability of combustion at high Mach numbers. We also want to see whether we are getting the expected thrust value,” Sivan said.

Although, as Shyam Mohan pointed out, the double-delta-wing configuration is the ideal candidate for hypersonic, supersonic and subsonic flight regimes, and the RLV configurations evolved over the years across the world have largely been around this shape, now most of the configurations are targeting what is called the drag minimum configuration. “Our present configuration is not a minimum drag configuration because of its wide wingspan,” Sivan told us. “All RLV designs abroad have a very slender structure, mainly to reduce drag. Once [after PFA] we have established that simulated data capabilities are exactly the same as what was seen in the flight, my understanding will be improved and I can use that modified data to go towards a correct design,” he said.

“If the RLV becomes viable, besides the direct spin-off of the cost of access to space coming down, from the satellite point of view, any in-orbit servicing in LEO, wherein something can be replaced, or extending the satellite’s life becomes possible,” said M. Annadurai, Director of the ISRO Satellite Centre (ISAC) in Bengaluru. “Towards that we are also planning some missions for docking. For that, technical feasibility exists, and we will be soon demonstrating that using a pair of small satellites,” he added.

“From the overall launch vehicle point of view, now our efforts will be to match the satellites to this new RLV capability. RLV possibly can be used for, say, four-tonne satellites. Even for communication satellites, the present scenario of DTH and other things call for higher power, and because the overall mass of the satellites has gone up, it is still beyond the capability of our present launch vehicles. It is possible to have a configuration called all EPS—all electric propulsion system—even with four-tonne satellites, when with RLVs we can realise 6-6.5 tonne in LEO because satellites need not carry any fuel. Also, in case manned mission comes, RLV will be useful. Technologically, RLV will enable manned missions,” Annadurai said. But the question that remains to be answered is whether an RLV, with a winged body and air-breathing, is feasible.

“In RLV, we always talk with air-breathing in mind. But, in general the thrust developed by air-breathing engines will be less than the conventional rocket engine thrust. We have to continuously accelerate and constantly build up the velocity from Mach 0 to even up to Mach 25. For this, the quantity T – D (thrust – drag) should always be positive. But since thrust cannot be increased much, drag has to be minimised, but that has been a challenge,” Sivan said. Even though many winged RLV designs were being evolved in the 1980s and 1990s, many of them have shut down or are shutting down because, as Sivan pointed out, the feasibility of T – D being positive all the time does not seem to be there with the present concepts. “Unfortunately, this is the state-of-the-art today,” said Sivan. (See also ISRO Chairman’s interview.)

Far from optimum So, notwithstanding the success of the first experimental flight, it is far from clear what final design the winged RLV will have because, as Sivan said, the present configuration is far from an optimum one. Further, given global developments—particularly by SpaceX, which is changing the very paradigm of approach to RLVs—it is also unclear at present where in ISRO’s scheme of things for future launch vehicles the RLV will be positioned in terms of its payload-carrying capacity.

Using its two-stage Falcon 9 rocket (which uses a cluster of nine semi-cryo engines in the first stage), SpaceX has launched satellites into orbit and also docked the Dragon capsule with the International Space Station. In all these launches, the first stage was brought back by ensuring that it had a soft landing. In one such launch, it made a vertical landing close to the launch pad that launched it. In two other recent flights, the first stage was, in fact, made to land vertically on a drone ship positioned in the sea. In this, the first stage, after separation, was flipped, reoriented and reignited so that the thrust during its descent would have a cushioning effect on the vehicle, enabling it to have a controlled descent and soft landing. The important thing here is that all this was done using existing concepts and technologies.

So, currently ISRO is grappling with the question, What should be its transportation system in the post-GSLV-MkIII phase? Irrespective of whether a winged RLV is realised or not (in any case, it is going to take at least another 20 years for the design to be perfected and all the technical issues to be resolved), in the short- and medium-term the organisation is looking at other options to bring down the launch costs, including recoverable technologies a la SpaceX’s heavy-lift (10-12 tonne) vehicles.

In the short-term, ISRO is looking at the possibility of using the semi-cryo engine, currently under development, in place of the twin Vikas liquid engine based L110 core stage of GSLV-MkIII. Its payload capacity will then increase from four to six tonnes. Increasing the payload fraction can also be done by choosing appropriate new structural materials as well as new technologies for sensors and the avionics package, such as MEMS (microelectromechanical systems), which render these very compact.

Sivan said: “As regards heavy-lift vehicles, we are in the process of discussion. It will be a TSTO vehicle whose first stage, like SpaceX’s, will be a five-engine semi-cryo cluster. This will have a modular structure. With one core semi-cryo stage, we can simply go on adding any number of strap-ons, and different payload requirements can be met. The second stage will be a cryo-stage. After the first stage is separated, it can be recovered in sea, like SpaceX has done. Though we have the technology to recover the cryogenic second stage, it is not very cost-effective. The first stage will not be a winged structure. We can manoeuvre it and bring it back to SHAR [in Sriharikota] also. But this will have a tremendous loss of payload. Because you have to retro-fire the rocket and have a controlled descent, you have to carry double the amount of propellants.”

However, Sivan points out that using our natural geographic advantage, the loss of payload can be minimised. “Since the vehicle, after separation, will, in its natural course, fall close to the Andamans, we can recover it there with a little manoeuvre. Other countries do not have this advantage. Now this seems to be more attractive than what was thought originally. Depending on the orbit, the second stage too can be brought back if required with a parachute. That is going to bring down the cost drastically,” Sivan said.

But Sivan feels that ultimately, for a real reduction in the cost of access to space, an improved winged first stage and a combination of propulsion modes which realise a positive T – D during the flight regimes involving air-breathing will be needed. ISRO itself is already working on an Advanced RLV (ARLV) concept. “The structure is very, very slim, not the present big wing-like structure. The vehicle will have semi-cryo boosters inside. Then there will also be a scramjet engine. And this is the upper cryo-stage. ARLV’s initial flight will be in rocket mode, then air-breathing mode and once again rocket mode. Then the second stage will take over. After separation, the first stage will be brought back. The concept is similar to what we were talking about initially. Only the shape and approach to propulsion are different.”

So what is the future of the winged RLV? After the successful maiden test launch, the debate is on among ISRO scientists.